Powder for laminated ceramic capacitor internal electrode

ABSTRACT

A metal alloy powder containing at least two alloying elements selected from the group of Ni, Cu, Cr, Sn, Mn, Co and W containing 1 to 99% by weight Ni, 1 to 99% by weight Cu, 6 to 60% by weight Cr, 6 to 15% by weight Sn, 6 to 15% by weight Mn, 6 to 15% by weight Co, and/or 6 to 15% by weight W for use in laminated ceramic capacitors with an internal electrode wherein said electrode comprises a sintered body of said alloy powder. A metal alloy powder containing at least two alloying elements selected from the group of Ni, Cu, Cr, Sn, Mn, Co and W wherein the onset of oxidation of the alloy powder occurs above about 250° C.

RELATED APPLICATION

[0001] This application is a continuation of International ApplicationNo. PCT/CA02/01585 filed Oct. 18, 2002, which is here incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to laminated ceramic capacitorsalso known as multi layered ceramic chip capacitors (MLCC), andparticularly to the internal electrode material used in the fabricationof such capacitors.

[0004] Multi layered ceramic chip capacitors generally consist of adielectric ceramic matrix with embedded metal sheet electrodes of someμm thickness and some 10 μm of distance. In manufacturing suchcapacitors, suitable pastes of powdered ceramic matrix precursormaterial and suitable pastes of a metal powder are alternativelylaminated on each other. Sometimes there is also provided a thinintermediate material. After lamination the laminate is dried and heatedto about 300 to 450° C. (normally under air) to decompose the organicbinder of the pastes. Thereafter the laminate is further heated undervacuum or inert gas atmosphere to about 1000 to 1350° C., mostly to atleast 1200° C., for sintering and formation of the ceramic dielectricmaterial.

[0005] During the decomposition step there is a risk that the metallicpowder of the internal electrode material is oxidized, which will bedeoxidized during heating to the sintering temperature. Deoxidationduring sintering leads to shrinkage of the internal electrode causingcracks and delamination of the capacitor and high percentages ofrejections from the manufacturing process.

[0006] Most of the multilayered ceramic chip capacitors use Pd or Pdalloys as the internal electrode material, which is sufficientlyresistant to oxidation, whereby deoxidation shrinkage is avoided.

[0007] Recent developments try to replace the precious Pd metal withBase Metal Electrode (BME) materials such as Ni or Cu with smallalloying additives such as Mg, Ca (U.S. Pat. No. 6,162,277 to Toshima etal.) or 95% Ni having at least one alloying additive of Mn, Cr, Co, Alor P (U.S. Pat. No. 5,319,517 to Nomura et al.).

OBJECTS OF THE INVENTION

[0008] An object of the present invention is to provide a particulatebase metal electrode material useful for internal electrodes oflaminated ceramic capacitors providing for an improved resistance tooxidation.

[0009] Another object of the present invention is to provide Base Metalpowders, which after sintering in an MLCC capacitor provide good or atleast acceptable electronic conductivity.

[0010] Another object of the present invention is to provide for amultilayer ceramic chip capacitor being less prone to cracking.

[0011] Another object of the invention is to reduce the number ofrejections of MLCC capacitors having Base Metal electrodes.

[0012] Other objects, advantages and features of the present inventionwill become more apparent upon reading of the following non-restrictivedescription of preferred embodiments thereof, given by way of exampleonly with reference to the accompanying drawings.

SUMMARY OF THE INVENTION

[0013] According to the broad concept presented, the present inventioncomprises alloy powders containing at least two alloying elementsselected from the group of Ni, Cu, Cr, Sn, Mn, Co and W comprising:

[0014] 1 to 99% by weight of nickel,

[0015] 1 to 99% by weight of copper,

[0016] 6 to 60% by weight of chromium,

[0017] 6 to 15% by weight of tin,

[0018] 6 to 15% by weight of manganese,

[0019] 6 to 15% by weight of cobalt, and/or

[0020] 6 to 15% by weight of tungsten.

[0021] Said alloy may additionally contain at least one of the elements(other than the at least two alloying elements mentioned above) selectedfrom Ag, Al, Au, B, Be, Ca, Ce, Co, Cr, Cu, Fe, Ge, Hf, Mg, La, Nb, Ni,Mn, Mo, Si, Sn, P, Pd, Pt, Ta, Ti, V, W, Y, Zn and Zr in an amount ofabout 0.1 to 20% by weight based on total metal. The additional alloyingelements may be present in an amount of 0.1 to 20% by weight based onthe total metal. Preferably at least two of the additional alloyingelements are present in the alloy powder. The total amount of additionalalloying elements in the alloy powder is preferably less than 6% byweight based on the total metal.

[0022] Preferred binary alloy powders comprise nickel-copper alloyshaving 1-99% by weight of nickel, more preferably 6-94% by weight ofnickel, particularly preferred 6-40% by weight of nickel or 6-40% byweight of copper and most preferred 15 to 30% by weight of Ni;nickel-chromium alloys having 6-60% by weight of chromium, morepreferably less than 40% by weight of chromium; copper-tin alloyscontaining 2 to 15% by weight of tin, more preferably 3-12% by weight oftin, and particularly preferred more than 6% by weight of tin.

[0023] The preferred binary alloys may become ternary or quarternaryalloys by including one or two of the additional alloying elements.

[0024] Preferred ternary alloy powders comprise copper-nickel-chromiumalloys containing 50 to 94% by weight Cu, 0.2 to 40% by weight Ni, and0.2 to 30% by weight Cr, preferably 60 to 90% by weight Cu, 2 to 25% byweight Ni, and 0.5 to 20% by weight Cr; copper-nickel-tin alloyscontaining 60 to 95% by weight Cu, 1 to 30% by weight Ni and 0.2 to 10%by weight tin, preferably 60 to 80% by weight Cu, 10 to 25% by weight Niand 2 to 10% by weight tin.

[0025] The preferred ternary alloys may become quarternary alloys byincluding one of the additional alloying elements.

[0026] The powders according to the invention preferably have a particlesize as derived from measurement of the specific surface area accordingto the BET method of 100 to 700 nm, preferably below 600 nm, morepreferably from 100 to 500 nm. For practical reasons resulting frommanufacturing methods of MLCC capacitors, presently particles sizes of250-400 nm are of particular use, however improved such methods whichare already experimentally in use will allow for use of powders in therange of 100 to 300 nm. Preferably the powders of the invention havesubstantially spherical shape.

[0027] Also, there is disclosed an alloy powder comprising copper and atleast one alloying element wherein the temperature at which oxidationoccurs is at least about 250° C., preferably between about 325° C. andabout 400° C.

[0028] Furthermore, there is disclosed a base metal alloy powdercomprising nickel and at least one alloying element wherein thetemperature at which onset of oxidation occurs is at least about 500°C., preferably between about 520° C. and about 600° C.

[0029] Additionally, there is disclosed a laminated ceramic capacitorwith an internal electrode wherein the electrode is fabricated from theabove alloy powders.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is a graph illustrating the increase in the temperature ofonset of oxidation between a base metal alloy powder fabricated usingcopper and a base metal alloy powder fabricated using a copper-nickelalloy;

[0031]FIG. 2 is a graph illustrating the results of an X-ray diffractionanalysis for the copper-nickel alloy powder of FIG. 1; and

[0032]FIG. 3 is a graph illustrating (i) the resistance to the onset ofoxidation, and (ii) the reduction in total shrinkage of an electrodefabricated using copper-nickel alloy powder as compared to thatmanufactured from a pure copper powder.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

[0033] An illustrative embodiment of a laminated ceramic capacitorinternal electrode material in accordance with the present inventionwill now be described.

[0034] The present invention concerns fine base metal alloy powders,typically having a mean particle size of below 1 μm, comprising binaryor ternary alloys of Ni, Cu, Cr and/or Sn and optionally containingminor amounts of additional alloying elements. The present inventionalso concerns capacitors whose internal electrodes are fabricated usingthese powders.

[0035] In general, fine metal and alloy powders are characterized by alarge surface area. Since the oxidation reaction takes place on theexterior surface of the individual particles, the greater the surfacearea of the powder, the more prone it is to oxidize. Oxidation reactionsare exothermic (i.e. they generate heat). Because of their very highsurface areas, fine powders tend to readily react with oxygen. Thisrapid oxidation results in a sudden local high temperature rise, whichin turn can lead to undesirable changes in sinterability anddelamination of the MLCC. Depending upon the selection of alloyingelements, in alloy powders, interdiffusion of the alloying elements willdepend on their respective affinity to oxygen and the permeability ofthe oxide layer formed. If the interdiffusion velocity of the alloyingelements is larger than the diffusion velocity of oxygen in the alloy,the metal of higher oxygen affinity may diffuse to the surface and fixoxygen at the surface, which leads to increased oxidation resistance ofthe powder. Maximum oxidation stability may occur at certain alloyingratio of the alloying elements and may not be achieved at completesegregation of the alloying elements. The situation is more complicatedin the case of more than two alloying elements due to the relativeinfluence of the alloying partners to each other as regards diffusivityand oxygen affinity.

[0036] The powders used in the fabrication of the internal electrode maybe manufactured according to any known method of manufacture of fineparticle powders, such as gel precipitation method with subsequentreduction of the precipitate, CVR methods by evaporation of suitablemetal compounds such as chlorides in hydrogen containing gas atmosphereand condensation of metal powder, evaporation or liquefaction of metalsin a plasma reactor and controlled condensation or solidification ofmetals. In an illustrative embodiment, the preferred method according tothe invention is a transferred arc plasma method as disclosed in WO00/10756 (corresponding to U.S. Pat. No. 6,379,419 to C. Celik et al.),the disclosure of which is included herein by reference.

[0037] Although most of these methods are disclosed for the manufactureof pure metal powders only, there is no principal difficulty inmodifying these methods to allow the manufacture of alloys of definedcomposition. For example, co-precipitation in the gel precipitationprocess is well known. In CVR methods either mixtures of the precursorcompounds may be evaporated from a single source or separate evaporatorsmay be used and the reactant gases may be introduced into the reactionand condensation tube via separate inlets into the tube.

[0038] According to a preferred process of manufacture fine metal alloypowders are produced by means of a transferred arc plasma system, whichprocess comprises the steps of: (a) continuously providing a mixture oralloy of metals to be vaporized in a transferred arc plasma reactor; (b)striking an arc between the metals and a non-consumable electrode in astraight polarity configuration to generate a plasma having atemperature sufficiently high to vaporize the metals and form a vaporthereof; (c) injecting a diluting gas heated to a temperature of atleast 1000° K into the plasma reactor; (d) transporting the vapor bymeans of the plasma gas and the diluting gas (both designated as carriergas) into a thermostatisized tube wherein the temperature is controlledat between 1000 and 1500° C. to control particle growth andcrystallization during passage of the carrier gas through the tube; (e)introducing the carrier gas with entrained alloy particles into a quenchtube with injection of a cooling fluid directly into the carrier gas,preferably in a sequence of cooling fluid inlets along the quench tube;(f) optionally introducing oxygen in an amount sufficient to effectsurface oxidation of the entrained alloy powders as an additive to thequench fluid supplied to at least at the first cooling fluid inlet; and(g) separating the powder particles from the carrier gas and the coolingfluid.

[0039] Preferably the plasma gas, diluting gas and cooling fluid areeither argon, nitrogen or another inert gas mixture with argon as thepreferred gas.

[0040] In the preferred transferred arc plasma method evaporation occursfrom a melt of the metal struck by the plasma arc. The melt has acomposition different from the composition of the desired alloy powderin order to compensate for different evaporation rates of the alloyingelements. Preferred is the continuous production method also disclosedin WO 00/10756, wherein the alloying elements are continuously fed intothe crucible of the plasma chamber at the desired alloying ratio,preferably in the form of prealloyed material.

[0041] Following start up and after expiry of an initial period of timethe composition of the melt will stabilize and assume a composition fromwhich the desired alloy evaporates. If the required melt composition isknown, the crucible may be filled at the start of production with acomposition from which the desired alloy composition evaporates. As isknown in the art, the required melt composition can roughly be estimatedfrom known vapor pressure versus temperature relations of the alloyingelements. Principally, it is also possible to have two or more plasmachambers from which the alloying elements are separately evaporated withintroduction of the gases into a common condensation and cooling tube.

[0042] Powders obtained by this preferred plasma process areparticularly low in sulfur, chlorine and carbon impurities. Thesepreferred products have chlorine contents of below 10 ppm, particularlypreferably below 5 ppm, and sulfur contents of below 25 ppm,particularly preferred below 10 ppm, and carbon contents of between 85and 600 ppm, preferably below 300 ppm.

[0043] The crucible from which the metals are evaporated preferablyconsists of zirconium oxide. The use of this crucible material resultsin a certain specific level of zirconium being present in themanufactured alloy powders of 15 to 175 ppm, preferably of up to 60 ppm.

[0044] A number of samples were prepared in order to illustrate theinvention. These examples are intended to disclose the invention in moredetail without limiting the generality of the disclosure hereof.

[0045] An experimental set up as disclosed in WO 00/10756 has been used.Prealloyed materials are filled into the crucible of the transferred arcsystem. The alloying ratio of the prealloyed precursor powder wasroughly selected at a modified ratio of the desired alloying ratiotaking into account different evaporation velocities of the elementsfrom the molten alloy. Argon was used as the plasma torch gas, thediluting gas and the cooling gas.

[0046] Table 1 shows the composition of the starting melts during thevarious runs. TABLE 1 Run Composition of starting melts in % by weight A85 Cu + 15 Sn B 30 Cu + 69 Ni + 1 (Al + Si) C 50 Cu + 50 Ni D 42 Cu + 58Ni E 95.1 Ni + 4.1 Cr + 0.8 Cu F 87.9 Ni + 11.9 Cr + 0.2 Cu G 73.6 Ni +26 Cr + 0.4 Cu H 99 Cu + 1 Mn J 91 Ni + 9 Co K 40 Ni + 60 W

[0047] Samples of pure copper and pure nickel were also used to providea comparative analysis.

[0048] The powders were sampled multiple times during each productionrun in order to determine the characteristics of the produced powders.Due to the non-optimized precursor composition for a specific desiredalloying ratio, differently composed alloy powders have been obtainedduring each single experiment.

[0049] The various powders obtained were analyzed for chemicalcomposition, particle size and the temperature of onset of oxidation.Particle size was derived from the gaseous absorption analysis methodknown under the acronym of BET (for Brunaner, Emmett and Teller, thediscovers of the method). The BET method is widely used for surface areadeterminations by computing the monolayer area. The temperature of onsetof oxidation was determined by differential thermal analysis (DTA). Theresults of the BET analysis and DTA are shown in the following tables.

[0050] It will be clear to one of ordinary skill in the art that,depending on their electro-negativity, all metals tend to oxidize tosome degree when exposed to air. This is due to the metal's propensityto donate electrons to whatever oxidant is present, in this case oxygenpresent in the air. Metals with a lower ionization potential such ascopper have a greater affinity to oxidants present and therefore quicklyoxidize. Although it is not clear the extent of the effects suchoxidation could have on the present invention, to ensure good qualityresults prudent practice would suggest proceeding with the fabricationof the laminated ceramic capacitor internal electrode shortly followingproduction of the alloy powders in order to limit the effects of thisoxidation. In any case, it is believed that if fabrication of theelectrodes is completed within one or two months of fabricating thepowders no adverse effects should be present. Additionally, shelf lifeof the powders could be extended by taking appropriate measures toensure that the powders are not exposed to oxidants.

[0051] In order to carry out the DTA a sample of the alloy powder wasplaced into a ceramic crucible which was in turn placed in an oven forheating. Air was injected into the oven at a constant rate and thesample gradually heated. As will be seen below, onset of oxidation canbe readily recognized as a rapid increase in the temperature of thesample, the rapid increase a result of the exothermic nature of theoxidation reaction.

[0052] Preferred powders according to the invention preferably show anonset of oxidation of at least about 325° C. Powders with copper as themajor alloying element are useful and feasible also with lowertemperature of onset of oxidation of at least about 250° C., althoughonset of oxidation temperatures of at least about 325° C. areparticularly preferred. It is difficult to achieve onset of oxidationtemperatures of above 400° C. with copper as the major alloying element.On the other hand, when nickel is used as the major alloying element,preferred onset of oxidation temperature is at least about 500° C.,particularly preferred at least about 520° C. although onset ofoxidation temperatures of up to 600° C. are easily achieved.

[0053] Table 2 shows the results for a series of copper metal powdersalloyed with different weights of a variety of other metals, includingtin, nickel and lesser amounts of aluminum and silicon. Table 3 showsthe results for a series of nickel metal powders alloyed with differentweights of chromium and copper. Finally, Table 4 discloses the resultsfor a copper metal powder alloyed with manganese and a nickel metalpowder alloyed with cobalt (J1) and tungsten (K1).

[0054] All samples had a chlorine content of about 3 ppm, a sulfurcontent of about 10 ppm, and a zirconium content of between 35 and 50ppm. TABLE 2 Powder Composition (% by weight) Particle Onset of Run/ CuSn Ni Al Si¹⁾ O C Size Oxidation sample wt % wt % wt % ppm ppm ppm ppmnm ° C. Pure Cu 100 — — — — 5500 150 407 180 100 — — — — 5500 150 541190 A1 89.3 10.7 <0.01 <9 200 6100 600 534 2) B1 82.2 — 15.2 560 10408135 687 193 161 B2 76.8 — 22 680 2150 4984 162 346 168 B3 73.5 — 25.1730 2250 5420 180 424 366 B4 70.2 — 28.3 780 2350 5850 197 415 398 B568.7 — 29.7 755 1800 7080 230 343 371 B6 67.3 — 31.1 730 1250 8300 267273 370 C1 81.2 — 18.5 4 1700 7220 200 339 327 D1 84.4 — 14.8 1 11005620 200 489 331 D2 83.1 — 16.5 6 1100 n/a n/a 482 343 D3 80.3 — 18.6 2200 8490 200 458 344 D4 80.8 — 18.8 8 1000 6840 200 545 357 D5 77.3 —22.3 9 1400 n/a n/a 541 359

[0055] TABLE 3 Powder Composition Onset of Run/ Ni Cr Cu O Sizeoxidation Sample wt % wt % wt % ppm nm ° C. pure 100 — — 2500 300360-350 Ni 100 — — to 400 380-400 100 — — 5000 500 400-420 100 — — 600450-500 E1 90.2 6.1 3.7 n/a 396 531 E2 88.4 6.3 4.6 4710 464 534 E3 90.36.7 2.4 4920 462 522 F1 86.9 10.9 0.6 n/a 436 528 F2 88 11 0.5 n/a 626551 G1 63.6 35.7 0.35 8780 606 >580 G2 61.5 37.3 0.8 9130 508 565 G358.4 39.4 1.66 12120 295 568

[0056] TABLE 4 Powder Composition Onset of run/ Ni Cu Mn Co W O SizeOxidation sample wt % wt % wt % wt % wt% ppm nm ° C. H1 — 88.7 10.9 — —7840 486 388 J1 87.8 — — 11.2 — 5180 510 568 K1 89.7 — — — 9.6 6330 473487

[0057] Referring now to FIG. 1, graphed results of the DTA for a purecopper (Cu) sample and a Copper-nickel alloy sample and clearlyillustrating the effects of the invention is disclosed. Apparent fromFIG. 1 is that the onset oxidation takes place at a much lowertemperature for the pure copper powder than that of the copper alloypowder. It will be apparent to one of ordinary skill in the art that thepeaks in temperatures of the powders indicated by the arrows are due tothe exothermic effects of the oxidation reaction and therefore serve asan indicator of the onset of oxidation.

[0058] Referring now to FIG. 2, samples of the powder were subjected tox-ray diffraction analysis, using Cu_(Kα) radiation. FIG. 2 shows suchtypical spectrum for sample E2, indicating excellent crystallinity ofthe powder. It will be apparent to one of ordinary skill in the art thatthe results graphed in FIG. 2 clearly show that a pure alloy forms andnot a composite of the two metals.

[0059] Referring now to FIG. 3, the relative change in length (i.e.shrinkage) of an electrode of a layered ceramic chip capacitor duringsintering, the electrode being fabricated from both alloyed andnon-alloyed powders is disclosed. The electrodes were fabricatedaccording to the parameters disclosed in Table 5. Particle Start of Endof Relative Change Powder Size Sintering Sintering in Length Pure Copper418 nm 250° C. 850° C. 21.0% Copper Nickel 489 nm 550-650° C. 850° C.19.3% Alloy (15% Ni) Copper Nickel 343 nm 500-600° C. 900-950° C. 17.7%Alloy (30% Ni)

[0060] It is apparent from FIG. 3 that the shrinkage experienced by bothalloyed electrodes is between 8 and 15% less than that for the purecopper electrode.

[0061] Although the present invention has been described hereinabove byway of a preferred embodiment thereof, this embodiment can be modifiedat will, within the scope of the appended claims, without departing fromthe spirit and nature of the subject invention.

1. A base metal alloy powder comprising an alloy including at least twoalloying elements selected from the group of Ni, Cu, Cr, Sn, Mn, Co andW wherein when present said elements are present in the followingamounts: 1 to 99% by weight Ni, 1 to 99% by weight Cu, 6 to 60% byweight Cr, 6 to 15% by weight Sn, 6 to 15% by weight Mn 6 to 15% byweight Co and 6 to 15% by weight W.
 2. An alloy powder according toclaim 1 wherein said alloy further comprises at least one of theelements selected from Ag, Al, Au, B, Be, Ca, Ce, Co, Cr, Cu, Fe, Ge,Hf, Mg, La, Nb, Ni, Mn, Mo, Si, Sn, P, Pd, Pt, Ta, Ti, V, W, Y, Zn andZr in an amount of about 0.1 to 20% by weight based on total metal. 3.An alloy powder according to claim 2 wherein said alloy furthercomprises at least two of said additional elements.
 4. An alloy powderaccording to claim 2, wherein the said additional elements are presentin said alloy in an amount of up to 6% by weight.
 5. An alloy powderaccording to claim 1, wherein said alloy is a nickel-chromium alloycomprising from about 6 to 40% by weight of chromium.
 6. An alloy powderaccording to claim 1, wherein said alloy is a nickel-copper-chromiumalloy comprising from about 0.2 to 30% by weight of copper.
 7. An alloypowder according to claim 6 wherein said alloy comprises from about 0.2to 30% by weight of chromium.
 8. An alloy powder according to claim 1,wherein said alloy is a copper-nickel-chromium alloy comprising fromabout 0.2 to 30% by weight of nickel.
 9. An alloy powder according toclaim 8 wherein said alloy comprises from about 0.2 to 30% by weight ofchromium.
 10. An alloy powder according to claim 1, wherein said alloyis a copper-tin-nickel alloy comprising about 1 to 30% by weight ofnickel.
 11. An alloy powder according to claim 1, wherein said alloycomprises at least 60% by weight of copper.
 12. An alloy powderaccording to claim 1, wherein said alloy comprises at least 60% byweight of nickel.
 13. An alloy powder according to claim 1 having anaverage particle size of about 25 nm to 700 nm.
 14. An alloy powderaccording to claim 13 having an average particle size of about 100 nm to700 nm.
 15. An alloy powder according to claim 1 having a substantiallyspherical shape.
 16. An alloy powder according to claim 1 comprisingcopper and at least one alloying element wherein the temperature atwhich onset of oxidation occurs is at least about 250° C.
 17. An alloypowder according to claim 16 comprising copper and at least one alloyingelement wherein the temperature at which onset of oxidation occurs isbetween about 325° C. and 400° C.
 18. An alloy powder according to claim1 comprising nickel and at least one alloying element wherein thetemperature at which onset of oxidation occurs is at least about 500° C.19. An alloy powder as in claim 18 wherein the temperature at whichonset of oxidation occurs is between about 520° C. and 600° C.
 20. Alaminated ceramic capacitor comprising an internal electrode fabricatedfrom an alloy powder according to claim 1.